1. Technical Field
The invention relates to an autofocus system, and more particularly to an autofocus system capable of performing focus adjustment while tracking a desired subject (person) by automatically moving an autofocus (AF) target position (range) in a shooting range of a camera.
2. Description of the Related Art
There has been known an AF area auto-tracking system that is adapted to change autofocus (AF) area in accordance with movement of a subject in order to bring the subject into focus when a quickly moving subject in sporting scenes such as a soccer and a figure skating is shot by, for example, a TV camera and the like (for example, see JP 2006-267221 A (corresponding to US 2006/0140612 A).
In an imaging system (like a TV camera) that converts an image of a subject to an electric signal (image signal) by an imaging device (CCD or the like), a contrast mode is generally adopted as an autofocus (AF) mode. The contrast mode is to detect the contrast of a subject image from an image signal acquired by an imaging device, and control focus of a taking lens so that the highest contrast can be obtained, thereby automatically bring the subject into best focus.
In the AF in the contrast mode, the entire subject in the shooting range of a camera is not set to the target of the AF, but only a subject in a pall of the shooting range is processed as the AF target in many cases. In the AF in the contrast mode, the image signals in the target range of the AF are extracted from the image signals acquired from the entire imaging area of the imaging device, and the target of the AF is limited only to the subject in a part of the range by controlling the focus so that the contrast of the extracted image signal in that range is maximized. In this specification, a subject range, being a target of the AF, in a shooting range of a camera or an image range of a target subject, being a target of the AF, in a shot image of a camera will be referred to as an “AF area”, and a frame indicating an outline of the AF area will be referred to as an “AF frame”.
In a lens system for broadcast, a cameraman performs a pan/tilt (P/T) operation and a zooming operation in order to capture a quickly moving subject like a figure skater. However, in such a situation, it is very difficult to move the AF area. In this case, the above-mentioned AF area auto-tracking system is useful.
However, for example, in the case where a soccer game is shot, for example, the case where a large number of subjects move similarly in a shooting range, it is difficult to continuously track a specific subject that is previously recognized by means of a simple pattern recognition technique, and the tracking target would be changed. As a result, there is a possibility that an image of the subject to be tracked originally might be blurred.
Furthermore, in a face recognition technique provided to recent digital cameras, it is effective to simply extract a person in a shooting range. However, in the case where not only a specific person but also a plurality of persons exist in a frame, it is difficult to recognize the specific person just by setting an AF frame onto a face. Thus, there is a possibility that the target of the AF may be changed to another person.